From: Tony Battersby Date: Fri, 13 Feb 2015 17:10:58 +0000 (-0500) Subject: sg: fix EWOULDBLOCK errors with scsi-mq X-Git-Tag: v3.19.1~77 X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=e1ed769da5d4910885850a104d910d0495738d71;p=thirdparty%2Fkernel%2Fstable.git sg: fix EWOULDBLOCK errors with scsi-mq commit 7772855a996ec6e16944b120ab5ce21050279821 upstream. With scsi-mq enabled, userspace programs can get unexpected EWOULDBLOCK (a.k.a. EAGAIN) errors when submitting commands to the SCSI generic driver. Fix by calling blk_get_request() with GFP_KERNEL instead of GFP_ATOMIC. Note: to avoid introducing a potential deadlock, this patch should be applied after the patch titled "sg: fix unkillable I/O wait deadlock with scsi-mq". Signed-off-by: Tony Battersby Acked-by: Douglas Gilbert Tested-by: Douglas Gilbert Signed-off-by: James Bottomley Signed-off-by: Greg Kroah-Hartman --- diff --git a/drivers/scsi/sg.c b/drivers/scsi/sg.c index 57178eda815d9..763bffe235174 100644 --- a/drivers/scsi/sg.c +++ b/drivers/scsi/sg.c @@ -1695,7 +1695,22 @@ sg_start_req(Sg_request *srp, unsigned char *cmd) return -ENOMEM; } - rq = blk_get_request(q, rw, GFP_ATOMIC); + /* + * NOTE + * + * With scsi-mq enabled, there are a fixed number of preallocated + * requests equal in number to shost->can_queue. If all of the + * preallocated requests are already in use, then using GFP_ATOMIC with + * blk_get_request() will return -EWOULDBLOCK, whereas using GFP_KERNEL + * will cause blk_get_request() to sleep until an active command + * completes, freeing up a request. Neither option is ideal, but + * GFP_KERNEL is the better choice to prevent userspace from getting an + * unexpected EWOULDBLOCK. + * + * With scsi-mq disabled, blk_get_request() with GFP_KERNEL usually + * does not sleep except under memory pressure. + */ + rq = blk_get_request(q, rw, GFP_KERNEL); if (IS_ERR(rq)) { kfree(long_cmdp); return PTR_ERR(rq);